Many devices for determining the distance to objects are known. One of the most currently used methods is called “Time Of Flight”. This method comprises sending a light signal towards the object and measuring the time taken by the signal to travel to the object and back. Generally, the calculation of the time taken by the signal for this travel is obtained by measuring the phase shift between the signal coming out of the light source and the signal reflected on the object and detected by a light sensor. Knowing this phase shift and the speed of light enables determination of the distance to the object.
FIG. 1 illustrates the general principle of the “Time Of Flight” method. In FIG. 1, a generator 10 (PULSE) provides a periodic electric signal, for example, square-shaped. This signal powers a light source 12. As an example, light source 12 may be a light-emitting diode, or any known lighting device, for example, a laser. The signal coming out of light source 12 is transmitted towards an object 16 and is reflected on this object to be detected by a light sensor 18, CAPT. The signal on sensor 18, CAPT, is thus phase-shifted from the signal provided by the generator by a time period proportional to twice the distance to object 16. Calculation means 20 (“DIFF”) receive the signals generated by generator 10 and by sensor 18 and calculate the phase shift between these signals to obtain the distance to object 16.
FIGS. 2A to 2C are timing diagrams illustrating the operation of a circuit such as that in FIG. 1. FIG. 2A illustrates a periodic signal “PULSE” capable of being provided by generator 10 of FIG. 1. FIG. 2B illustrates the signal received by sensor 18, CAPT. Due to interactions with the outside and to the components forming sensor 18, the signal received by this sensor has, in this example, variations in the form of capacitor charges and discharges. The signal on sensor 18 is phase-shifted from the signal coming out of generator 10 by a delay D.
Usually, sensor 18 integrates one or several photodetection elements enabling one to efficiently detect the signal received after reflection on object 16. Such elements conventionally are rapid charge transfer photodiodes. Single-photon avalanche diodes, or “SPADs”, also called Geiger diodes, may also be used. FIG. 2C illustrates the signal (PULSEC) generated by sensor 18, in the case where this sensor contains such a SPAD.
SPADs operate as follows. At an initial time, the diode is biased to a voltage smaller than its breakdown voltage. The reception of a photon in the diode junction area starts an avalanche in the diode, which creates an electric pulse. The diode is then biased back to a voltage smaller than the breakdown voltage, so that it reacts again to the reception of a photon. SPADs can currently be used in cycles having reactivation periods shorter than 10 ns. Thereby, they can be used at high frequency to detect objects at relatively short distances from the measurement device, for example, distances ranging from a few centimeters to a few tens of centimeters.
As illustrated in FIG. 2C, a disadvantage of SPADs is that, if they receive a light signal such as described in relation with FIG. 2B, the diode avalanche time may slightly vary with respect to this signal. Indeed, the histogram of the number of pulses versus time reflects the power-time profile of the light received by the SPAD. Thus, in the case illustrated in FIGS. 2A to 2C, on a large number of acquisitions, the histogram of the pulse transmission by the SPAD substantially follows the curve of FIG. 2B. The determination of the distance to an object based on the information relative to the delay between the signal transmitted by source 12 and the generation of a pulse by the SPAD is thus not reliable if it is carried out over a single period.
Digital counter devices intended for a histogram comparison, associated with SPAD sensors, are known. However, such devices are relatively complex to implement.
The use of sensors comprising photodiodes of charge transfer on several nodes according to the phase of the signal transmitted by the reference generator is also known. The comparison of the amount of photogenerated charges in phase with the reference signal and phase-shifted therewith provides information as to the distance to the object. However, such devices induce an error in the distance estimate, which may be critical for short distances.
Further, a general disadvantage of known devices is that they are generally sensitive to the ratio between the light used for the detection and the ambient light. Indeed, such devices lose much accuracy when the ambient light is strong with respect to the useful light. Known devices are further sensitive to the waveform of the light emitted by the generator, which is hardly ideal.
There further is a need for a method for determining the distance to the object, which is efficient for short distances and overcomes the disadvantages of known devices.